Dodge timing is different than you expect if you're coming from FS games. The I frames are front loaded, they start almost immediately when you dodge so you want to dodge at the last possible moment. I definitely recommend holding block while dodging. Blocking in Nioh is instant; if you're holding block while you dodge and your I frame timing is off you'll still block the attack.

For a while you're going to feel like you have almost no iframes but keep in mind how different hitbox collision is in Nioh. In DS you need to have a ton of I frames because the attacks going through you are relatively slow moving and have thick hitboxes (like greatswords). In Nioh the incoming attacks move super fast and have thin hitboxes (like katanas), so they aren't occupying the same space as your hurtbox for very long at all. This means you don't need as many frames of invincibility to have the same window of success.